It has long been known that the reduction of iron ore to produce iron in a form suitable for steel-making can be carried out at temperatures below the melting point of iron by passing a hot reducing gas through a bed of particulate iron ore at temperatures of the order of 700.degree. to 1000.degree. C. to produce sponge iron. Because of its wide availability, natural gas, (typically reported as containing 75%, 85%, or more of methane) has been extensively used as a source of the reducing gas for such sponge iron processes. It is well known that natural gas also contains lesser amounts of other simple hydrocarbon homologs, such as ethane (on the order of 10%), propane, butane, and even pentane, etc. Thus, constituent percentages range considerably, dependent upon the particular source. The reader is referred to the literature, where it has been found that natural gas does not contain any more than trace amounts of hydrogen or carbon monoxide (being substantially less than 1%). However, since methane per se and the other paraffin hydrocarbons of a low molecular weight present in natural gas (whether alone or in combination), are relatively ineffective reducing agents for iron ore, it has been customary to convert the methane and/or the other lower hydrocarbons into a mixture of carbon monoxide and hydrogen for use as ore-reducing agents. More particularly, a mixture of natural gas and steam is catalytically converted into a carbon monoxide and hydrogen mixture in a reformer and the resulting upgraded reducing gas mixture is heated, if necessary, and passed through a bed of particulate iron ore to convert it to sponge iron.
Typical gaseous reduction systems are disclosed, for example, in U.S. Pat. Nos. 3,765,872; 4,099,962; 4,150,972; 3,748,120; 4,046,557 and 3,905,806. Such systems commonly comprise vertical reactors having a reducing zone in the upper portion thereof wherein the hot reducing gas flows upwardly counter-current to a descending body of iron ore, and a cooling zone in which the reduced ore in the form of sponge iron is cooled with a cooling gas. The spent reducing gas removed from the top of the reducing zone is de-watered, mixed with fresh reducing gas from the reformer, reheated and recycled to the reducing zone of the reactor.
While reduction systems using reformed natural gas as a reducing agent have been extensively used commercially, they are open to the serious objection that the catalytic reformers they employ are costly pieces of equipment and form a substantial part of the investment in such a sponge iron producing plant. Hence a process capable of producing high quality sponge iron without using an external reformer would substantially reduce the capital cost of such a plant.
As indicated above, natural gas per se is an unsatisfactory reducing agent for use in an iron ore reduction reactor for a number of reasons. Thus the reduction reaction rate using a gas containing mainly methane and/or its homologs as the reducing agents is substantially less, at a given temperature, than the reaction rate for mixtures of carbon monoxide and hydrogen. While it is true that the reaction rates for methane and its homologs can be increased by raising the temperature, the higher temperatures required to achieve an acceptable reaction rate lead to other problems. Thus at temperatures above about 1000.degree. C. methane decomposes, especially in the presence of iron, to form solid carbon in the form of soot that coats the iron-bearing material and restricts access of the gas to the interior of the particles or pellets to be reduced.
Moreover, at such elevated temperatures there is a tendency for the reduced ore to sinter and agglomerate into large aggregates. In order to achieve acceptable operation of a vertical shaft, moving bed reactor, a free flow of the particles or pellets through the reactor, and particularly through any reduced cross-sectional areas of the reactor, is essential. The formation of large irregular aggregates, can in some cases, completely block the flow of solids in the reactor and also cause undesirable channeling of the gas flow therethrough.
Still further, the reduction reactions that occur in the reactor are generally endothermic and hence the feed gas must be heated outside the reactor to provide the necessary reaction heat. The use of high reaction temperatures increases the reducing gas heating costs.
One effort to solve the foregoing problems is disclosed in Kaneko et al. U.S. Pat. No. 4,268,303. In accordance with the disclosure of this patent the reduction of the ore is carried out in two stages. In the first reduction stage, methane gas is used as a make-up gas and the reduction is carried out to the point where a metallization of 30% to 80% is attained. The final reduction up to about 95-98% is achieved in a second stage wherein a reformed gas largely composed of carbon monoxide and hydrogen is employed. Since no reformer is needed to produce the first stage reducing gas, the reformer can be relatively small. However, a reformer to produce the second stage reducing gas is still required if an acceptably high metallization is to be attained. A disadvantage of the process of this patent is the extremely high capital costs, because of the higher operating temperature of the gas heater. Another disadvantage is the greater heat losses inherent in operating at such high temperatures.
It has been generally recognized in this art that methane and its homologs can not, as a practical matter, be used directly per se without prior conversion to H.sub.2 and CO in the direct reduction of iron ore. For example, it is stated in the article "the reduction of iron oxides in a methane gas uniflow y A. Domsa and Z. Sparchez of the Polytechnic Institute Cluj, Bulgaria, (Cercetari Metalurgice, Vol. 9, 1967, p. 133-141), that methane can reduce iron ore effectively only at elevated temperatures, above 1000.degree. C. Such theoretical use at temperatures above 1000.degree. C. is not practical because of the agglomeration problems discussed elsewhere. Contrary to this teaching, the applicants have discovered that a steady state direct "addition of methane (typically in the form of natural gas) can be achieved in the temperature"; range of 800.degree. to 1000.degree. C., more preferably from 900.degree. to 960.degree. C.